Conventionally, there is known a surface magnet type motor including a rotor having a magnet arranged at the outer circumferential surface of the rotational shaft. This type of motor is provided with a non-magnetic anti-scattering tube at the circumference of the magnet to prevent fracture and scattering of the magnet caused by the centrifugal force when the rotor rotates at high speed.
The anti-scattering tube of the motor is generally fixed by press-fitting the magnet into the inner cylinder of the anti-scattering tube. Accordingly, the magnet is firmly maintained with force to prevent fracture and scattering of the magnet caused by centrifugal force (for example, refer to Japanese Patent Laying-Open Nos. 2001-025193 and 2001-218403).
In order to realize usage under a high centrifugal force in a wide range of operating temperature to accommodate higher speed rotation of a motor having such a structure, the press-fit margin between the anti-scattering tube and magnet must be increased to hold the magnet more firmly with stronger force. However, increase of the press-fit margin induces the possibility of the magnet being fractured during the press-fitting stage into the anti-scattering tube.
Even if the event of fracture does not occur at the time of assembly, such stress on the magnet may result in fracture during operation of the surface magnet type motor by the effect of the centrifugal force. Particularly in the case where such a motor is employed for the assist of the turbine shaft of a turbocharger that rotates as fast as one hundred and several ten thousands to two hundred and several ten thousands rpm, the strength as high as the operation limit of the turbine shaft must be ensured. Thus, the need arises for a structure and/or fabrication method that alleviates the stress on the magnet.